The present invention relates to optical aliasing filters, pixel sensor arrangements from a pixel sensor and an optical aliasing filter and pixel recording devices, like e.g. digital cameras.
Image sensors (CCD or CMOS) consist of individual light-sensitive pixels, arranged in a regular grid. The image projected to the sensor is thus divided into image points or pixels. By this sampling of the image in the two-dimensional range, aliasing effects result, when spatial frequencies with more than half the sampling frequency (spatial frequency of the pixels) exist in the image. Half the sampling frequency, as also in telecommunications, is referred to as Nyquist frequency.
In general, the “bandwidth” of the spatial frequencies in the projected image is delimited by mapping characteristics of the lens. In this connection, this is referred to as the modulation transfer function of the lens or objective, in the following referred to as MTF. It is the representation of the transmission function in the frequency range. The corresponding representation of the transmission function in the spatial range is referred to as “point-spread-function”, briefly PSF.
With high-quality lenses, the MTF exceeds the Nyquist frequency of the sensor so that more or less strong aliasing effects result. These manifest themselves in the electronic image in the form of moiré-like patterns and colored artifacts.
For a defined limitation of the bandwidth of the spatial frequency in the image, so-called aliasing filters (also optical low pass filters; OLPF) are used. The aliasing filter is inserted between the lens and the sensor in the optical path, i.e. generally in parallel to the sensor surface.
The functional principle of conventional aliasing filters is based on the physical effect of optical birefringence. In detail, one or several plane-parallel plates made of optically active materials are used arranged one after the other and which are generally cemented. As a material often mono-crystalline quartz crystals are used; other materials are also possible. The plates are cut out of the crystal in a certain direction, so that a lightbeam penetrating the plate is split into an ordinary and an extraordinary lightbeam. Both partial beams pass divergently in the medium and are again aligned in parallel by birefringence after exiting the medium. In this way, two partial beams resulted from one beam. The interval between the two partial beams depends on the optical characteristics of the applied material and on the thickness of the plates. As the partial beams are parallel, the effect of the aliasing filter is independent of the location between lens and image plane (sensor area).
If more than one plate is used, and if the further plates are rotated with regard to the orientation of the optical axis by certain angles with regard to the first plate, then the partial beams resulting in the first plate are successively further divided. Here, in general 2n partial beams result, wherein n is the number of plates.
By dividing the original beam into two or several partial beams, for the aliasing filter a transmission function results, which together with the MTF of the lens and the sensor leads to a more or less strong suppression of spatial frequencies above the Nyquist frequency. By a suitable implementation of the aliasing filter, aliasing effects within the image may thus be weakened specifically. Here, however, inevitably, the sharpness in the image is reduced.
In order for the aliasing filter to have a good effect, the bundle of partial beams has to have a diameter which approximately corresponds to the distance of the pixel centers on the sensor. With large pixels, as they are used in high-quality cameras, thus the distance between the partial beams has to be correspondingly large. In this respect, either relatively thick plates of the conventional material quartz are needed or plates made of special materials are used comprising a stronger birefringence. These materials are, however, substantially more expensive than quartz and more difficult to process.
It would be desirable to have a concept for an anti-aliasing filtering which may be integrated into optical systems more easily and with less effort and/or may be realized more cheaply.
Apart from that, realizations of aliasing filters exist, which are based on diffraction, as it is for example described in U.S. Pat. No. 6,040,857, U.S. Pat. No. 6,326,998, U.S. Pat. No. 4,178,611, U.S. Pat. No. 4,998,800 and U.S. Pat. No. 4,878,737, but due to the way of diffraction these solutions lead to a wavelength dependent aliasing or low-pass filtering, which leads to image quality degradation.